(i) Field of the Invention
The present invention relates to a transparent panel heater which can be used in a window portion. More particularly, it relates to a transparent panel heater which can be used in liquid crystal displays, refrigerated showcases, freezer showcases, defrosters for windshields, and the like.
(ii) Description of the Related Art
In a freezer showcase or a refrigerated showcase, it is necessary to prevent dew from condensing on the surface of a glass plate constituting its window. Therefore, a transparent conductive film has been formed on the glass surface, and predetermined electric power has been applied to the transparent conductive film to heat the glass surface.
In recent years, the demand of liquid crystal displays has increased, but when they are used in a cold environment, their operation is inconveniently slow. Thus, there has also been an increased need for transparent panel heaters for temperature control with liquid crystal displays.
Heretofore, in a liquid crystal display which can be used in a cold environment, a mesh-like heating resistor is disposed to heat it, as suggested in, for example, Japanese Patent Application Laid-open No. 126517/1983. In this constitution, however, it is difficult to uniformly heat the whole liquid crystal display, and the heating resistor comprising an opaque metal tends to inconveniently disturb an operator in watching the display.
A transparent heating element in which a transparent conductive film is formed on a transparent substrate has been suggested in, for example, U.S. Pat. No. 4,952,783. The constitution of such a heating element is shown in FIG. 1 attached hereto. That is, a transparent conductive film 52 is formed all over the surface of a transparent substrate 51, and a pair of electrodes 53 for feeding electric power to the transparent conductive film 52 are arranged at both the ends of the transparent conductive film 52. Furthermore, in order to protect the transparent conductive film 52 and the electrodes 53, a transparent protective layer 54 is formed all over the surface of the heating element. Here, the electrodes 53 are formed by applying a conductive printing material such as a silver paste on suitable positions of the transparent conductive film 52 by a screen printing method, and then thermally treating the applied coating material. For the purpose of improving the reliability of the electrodes, Japanese Patent Application Laid-open No. 289685/1992 discloses electrodes constituted by sandwiching a metal foil with electrically conductive printing material layers.
However, where the electrodes are constituted by an electrically conductive printing material such as the silver paste in this kind of transparent panel heater, the resistance of the conductive printing material itself is larger than that of the transparent conductive film, and the contact resistance between the electrodes and the transparent conductive film tends to increase. If the contact resistance increases, a conduction state in the transparent conductive film is uneven with the enlargement of the transparent panel heater. As a result, heat is unevenly generated, and the transparent panel heater is uniformly heated on the whole. In addition, current is concentrated in the vicinity of the contacts of the electrodes, so that portions of the transparent panel heater in the vicinity of the electrodes are abnormally heated and disconnection tends to inconveniently occur. In the case of such electrodes as disclosed in the above-mentioned Japanese Patent Application Laid-open No. 289685/1992, the unevenness of the conduction state can be solved, but adhesive properties between the transparent conductive film and the electrodes are poor and both of them are liable to peel from each other. In addition, a manufacturing process for the formation of such electrodes is intricate and their workability is low, which increases the cost of products.